1. Field of the Invention
The present invention relates to an endoscope system and endoscope operating method. More particularly, the present invention relates to an endoscope system and endoscope operating method in which pixels of an image sensor are corrected by gain correction, and a difference in spectral sensitivity between pixels can be compensated for easily and appropriately.
2. Description Related to the Prior Art
An endoscope system is well-known in the field of medical diagnosis, and includes a light source apparatus, an electronic endoscope and a processing apparatus. The light source apparatus emits light for illumination to an object of interest in a body cavity. An image sensor in the endoscope images the object of interest illuminated with the light, and generates an image signal. The processing apparatus processes the image signal in image processing, and generates an image for display on a monitor display panel.
Two lighting controls are available in the endoscope system, including field sequential lighting and simultaneous lighting. In the field sequential lighting, red (R), green (G) and blue (B) light components are applied to the object of interest sequentially one after another. A monochromatic image sensor images the object of interest illuminated with the light components in sequential steps. In the simultaneous lighting, red (R), green (G) and blue (B) light components are applied to the object of interest simultaneously, so that white light is applied thereto. A multi-color image sensor for use with simultaneous lighting is used, has a color filter, and images the object of interest illuminated with the white light.
The field sequential lighting generates one image by imaging of three frames with the monochromatic image sensor, and has a feature of high spatial resolution and low time resolution. In contrast, the simultaneous lighting generates one image by imaging of one frame with the multi-color image sensor, and has a feature of high time resolution and low spatial resolution. In short, the field sequential lighting and the simultaneous lighting have the features distinct from one another. There is a known endoscope system in which a first endoscope and a second endoscope are selectively connectable to a light source apparatus and a processing apparatus, the first endoscope having the monochromatic image sensor in combination with the field sequential lighting, the second endoscope having the multi-color image sensor in combination with the simultaneous lighting.
It is necessary in an endoscope system to prepare the first and second endoscope. JP-A 2009-284931 and JP-A 2009-284959 suggest a structure in which a lighting control is changeable over between simultaneous lighting and field sequential lighting while the second endoscope is kept connected. In the patent documents, gain correction is performed for a pixel signal of a pixel of a low sensitivity for the light of the plural colors in the pixel signals generated by a multi-color image sensor (for use with simultaneous lighting) in a state of the field sequential lighting for the lighting control. The pixel signal of the pixel of he low sensitivity is used for generating an image together with a pixel signal of a pixel of a high sensitivity, so that spatial resolution can be increased.
A gain value for use in the gain correction is determined by test imaging of a white object with the multi-color image sensor in the field sequential lighting, and by adjusting the white balance in such a manner that a sum of pixel signals of one pixel obtained by lighting of red, green and blue light becomes a value of a pixel signal of white. To be precise, a ratio of a value of pixel signal obtained with first light component of highest sensitivity to a value of pixel signal obtained with remaining light components is obtained in relation to the red, green and blue light components is obtained for each one of pixels of the multi-color image sensor. Then the gain value is determined by obtaining the ratio. For example, the ratio is that of a pixel signal of a red pixel according to lighting with red light to a pixel signal of a red pixel according to lighting with green or blue light.
In general, the pixel signal P is proportional to a value of integration of a product of multiplication of a spectrum I(λ) of light intensity of light of illumination, a spectrum S(λ) of spectral sensitivity of a pixel, and a spectrum R(λ) of spectral reflection of body tissue, from minimum wavelength λmin to maximum wavelength λmax of the spectrum I (λ) of the light intensity. See the mathematical relationship (1).P∝∫λminλmax I(λ)·S(λ)·R(λ)dλ  (1)
In the endoscope system of JP-A 2009-284931 and JP-A 2009-284959, the gain value is determined by obtaining a ratio between values of pixel signals between plural light components with a difference in the wavelength range. Spectral reflectance of body tissue on which values of the pixel signals are dependent is different between the wavelength ranges of the light components. The differences in the spectral reflectance between the wavelength ranges differ between plural samples of the body tissue. See FIG. 14. The gain value must be determined again at each time that the body tissue of imaging is changed.
In the endoscope system of those patent documents, a width of the wavelength range of each one light components (difference between λmax and λmin) is considerably large. Dependency of the spectral reflectance of body tissue or object or interest to the wavelength is not negligible within the wavelength range of the light components. Pixel signals are influenced by the spectral reflectance of the body tissue. It is still necessary to redetermine the gain value for body tissue due to the additional reason.